1. Field of the Invention
The present invention relates to an electronic flash device, and particularly to an electronic flash device capable of changing the illuminating angle.
2. Description of the Prior Art
Until now, in order to adjust the illuminating angle of a flash device according to the focal length of a photographic lens, a Fresnel lens is arranged in front of a light emitting part to be movable in the direction of the optical axis. The size of the flash device is remarkably large as compared with a flash device incapable of changing the illuminating angle. It is inconvenient for handling. Although the flash device of this kind can cover the picture angle of photographic lenses of 28 mm to 85 mm for a 35 mm camera, the device of the same shape is not fit for a lens of wider angle, for example, 24 mm lens. Therefore, it is necessary to mount a wide adapter with dispersion characteristics on the flash device. It makes the handling all the more complicated.
FIG. 3 shows a sectional view of the conventional flash device. In the drawing, 1 is a flash discharge tube, 2 is a back metal formed of metal for radiating the heat from the discharge tube 1. 3 is a reflector formed of plastics. The back metal 2 and the reflector 3 are combined to form a reflector almost in the shape of ellipse. The discharge tube 1 is arranged in the neighborhood of the focal point of the reflector. 4 is a dispersing plate for homogenizing light emitted from the discharge tube 1 and adjusting color of light emitted from the discharge tube 1 so as to have a spectrum similar to that of the sun beam. The back metal 2, the reflector 3 and the dispersing plate 4 are secured to the body 5.
6 is a Fresnel lens whose Fresnel plane is formed at the side opposite to the light emitting part. The Fresnel lens 6 is secured to a movable frame 7 and is movable in the direction of the optical axis according to the focal length of a photographic lens, changing the illuminating angle. In FIG. 3, a position of the Fresnel lens 6 shown by the solid line corresponds to the picture angle of a 28 mm lens, while the position shown by the two-dot-dash line corresponds to the picture angle of an 85 mm lens. The reflector is almost of elliptical shape as mentioned above. The shape of this conventional reflector is represented as follows: ##EQU1## where 2a is a longer diameter of the reflector, and 2b is a shorter diameter of the reflector. Further, as the Fresnel lens 6, a panel with the focal length of about 60 mm is often used. The moving distance of the Fresnel lens 6 is about 20 mm, so that the illuminating angle covering the lenses of 28 mm to 85 mm can be obtained.
The above construction can cover the picture angle of the 28 mm lens, as mentioned above, for the lenses of wider angle it is necessary to mount the adapter such as the wide panel in order to enlarge the illuminating angle. It makes the handling complicated.